The present invention relates to a process and a plant for separating air by cryogenic distillation, and in particular a process for producing pressurized gaseous oxygen, and possibly gaseous nitrogen, using a single column.
Since the beginning of the century, air distillation has been carried out in a double column comprising a medium-pressure column and a low-pressure column connected by a heat exchanger.
Several solutions have been proposed in various patents to reduce the number of columns from two to one.
U.S. Pat. No. 4,947,649 describes a solution in which air is compressed before being at least partially introduced into a single column. Such a solution is applicable only if it is desired to produce nitrogen at a pressure substantially higher than atmospheric pressure, especially in the case of integration with a gas turbine. On the other hand, if the pressure of the air delivered by the gas turbine compressor is very high, it is hardly recommendable to use this process since the distillation at high pressure (a pressure above 15 bar) is very difficult and poses not insignificant technological problems when the pressure approaches the supercritical pressure of nitrogen (33 bar). The other drawback of the cycle described in that patent is that the gaseous oxygen is produced at the same pressure as the air sent into the single column.
EP-A-0 584 420 relates to a single column which produces oxygen and nitrogen using a top condenser and two reboilers operating at between 5 and 20 bar. One of the reboilers is warmed with compressed nitrogen at ambient temperature and then cooled.
Patent EP-B-0 606 027 also describes a single-column process for producing pressurized oxygen and/or nitrogen, together with at least one liquid product. Such a process is not beneficial if it is not desired to produce liquid products since the air pressure depends eminently on the amount of liquid produced. When producing no or little liquid, the air pressure is less than 3 bar abs, which poses problems in the design of the top purification, which requires an enormous amount of absorbent, making this process uneconomic. U.S. Pat. No. 5,794,458 also describes an air distillation process using a single column. The main criticism that can be levelled at such an arrangement is that it includes a cold compressor which compresses a fluid very rich in oxygen. Moreover, the compression of the air is conventionally carried out in one or more compressors operating at ambient temperature.
DE-A-1 199 293 describes an air distillation process in which a stream of air is separated in a single column and a stream of liquid oxygen is withdrawn from the bottom of the column and vaporized by heat exchange with a stream of cycle nitrogen compressed in a cold compressor. A portion of the nitrogen compressed in the cold compressor to between 30 and 40 atma serves to reboil the single column. In this case, it is necessary to warm the nitrogen in order to compress it before cooling it and liquefying it against the oxygen which vaporizes. This is expensive in terms of energy and complicates the construction of the exchangers.
U.S. Pat. No. 5,475,980 describes a double-column air distillation process which, in a novel manner, proposes to compress a portion of the air needed for the distillation in a cold compressor. The drawback of such a solution is the complexity of the exchange line from which the cold fluid to be compressed is withdrawn before reintroducing it therein.
In the air distillation processes according to the invention using a single column, a cold compressor compresses a fluid whose oxygen content does not exceed 30 mol % . Another advantage of such an arrangement is that it is better in terms of energy than the arrangement described in U.S. Pat. No. 5,794,458 since the turbine of the invention, working on a fluid entering the cold box and not a fluid leaving the cold box, the amount of heat exchanged in the main exchanger is markedly less, and hence there are fewer irreversibilities. Another aspect of the invention is that it produces oxygen at a pressure greater than the pressure of the single column by compressing an oxygen-rich liquid (either by pump or by net positive suction head) at a pressure greater than that of the single column and by vaporizing it either by indirect heat exchange in a main exchanger or an external vaporizer, or by direct contact in a mixing column. Finally, the coproduction of liquid products in addition to the gas products is not necessary in order to make this process attractive, even though it is possible.
The ambient temperature is defined by the temperature at the intake of the main air compressor for feeding the separation unit.
According to the invention, a process for separating air by cryogenic distillation is provided, comprising the steps of:
compressing the air, purifying it and sending at least one portion thereof to a first (or the) column;
separating air in the column at cryogenic temperature;
compressing at least one portion of a fraction containing at most 30 mol % of oxygen extracted from the column in a compressor, the intake temperature of which is below room temperature;
at least partially cooling the said compressed fraction and condensing it by vaporizing an internal fluid of the first column or a fluid extracted therefrom, and possibly after having enriched it with nitrogen; and
extracting an oxygen-rich liquid fraction from the first column, pressurizing it to a pressure above that of the column and vaporizing it by direct or indirect heat exchange with a portion of the feed air in order to form an oxygen-rich pressurized gas product.
According to other aspects of the invention:
a nitrogen-rich gas product is withdrawn from the top of the first (or the) column;
a fraction containing at most 30 mol % of oxygen extracted from the column is compressed in a compressor whose intake temperature is below the ambient temperature to a pressure of less than 30 bar abs;
the pressure in the first (or the) column is between 1.3 and 20 bar abs, preferably between 3 and 10 bar abs;
the compressed fraction contains at most 19 mol % of oxygen and at least 81 mol % of nitrogen, preferably at least 90 mol % of nitrogen;
at least one portion of the air is expanded in a turbine before being sent to the first (or the) column;
the production of work by the expansion of at least one portion of the air is at least partially used to compress the fraction containing at most 30% oxygen in one or more compression stages;
at least one portion of the air is compressed to a high pressure, condensed and sent to the first (or the) column;
an unexpanded portion of the air is condensed by vaporizing an internal fluid of the first column or a fluid withdrawn therefrom (FIG. 2);
the oxygen-rich liquid fraction is vaporized by direct contact in an auxiliary column called a mixing column (FIG. 3);
an auxiliary column intended for argon production is fed from the first column (FIG. 4);
an oxygen-enriched liquid withdrawn from the single column is distilled in an auxiliary column in order to produce a fraction richer in oxygen and a fraction depleted in oxygen, both fractions being reintroduced into the first column (FIG. 5);
at least one portion of the air intended for a column of the apparatus comes from the compressor of a gas turbine and/or a nitrogen-enriched gas coming from the first (or the) column is sent back to the gas turbine system;
the inlet pressure of the gas turbine is greater than 15 bar abs;
the purity of the gaseous oxygen produced is at least 80 mol %, preferably at least 90 mol %;
the intake temperature of the cold compressor is below xe2x88x92100xc2x0 C. or preferably below xe2x88x92150xc2x0 C.;
liquid may or may not be produced as final product;
the compressed fraction at least partially condenses in the bottom reboiler of the first (or the) column;
the stream of air which is used to vaporize the oxygen-rich liquid at least partially condenses and is sent to the first column;
the compressed fraction is enriched with nitrogen in a distillation column thermally coupled to the first column.
According to another aspect of the invention, a plant for separating air by distillation in at least a first column is provided, this column having a bottom reboiler, comprising means for sending compressed and purified air to the column, a compressor for compressing a gas containing at most 30 mol % of oxygen coming from the column having an inlet temperature of at most 5xc2x0 C. warmer than a temperature of the column, optionally means for enriching the compressed gas with nitrogen upstream of the reboiler, means for sending the compressed gas to the bottom reboiler, means for sending the at least partially compressed gas condensed in the bottom reboiler back to the column, means for withdrawing an oxygen-enriched liquid from the bottom of the first column, means for pressurizing it and means for vaporizing the pressurized liquid by heat exchange, in order to form an oxygen-rich pressurized gas product, characterized in that it includes means for vaporizing the pressurized liquid by direct or indirect heat exchange and, if the exchange is indirect, the heat exchange takes place with air intended for the first column.
According to other inventive aspects:
the apparatus comprises a turbine fed with air and the outlet of the turbine is connected to the first column;
the pressurized liquid vaporizes in a mixing column;
the apparatus comprises an argon production column fed from the first column having a bottom reboiler;
the column having a bottom reboiler has at least one intermediate condenser;
the column having a bottom reboiler does not have a top condenser;
there is a second column thermally coupled to the first column, which optionally includes means for sending the overhead gas from the second column to the bottom reboiler;
there are means for sending the gas compressed in the compressor into the bottom of the second column.